Heat oven to 400°F. In ungreased 15x10x1-inch pan, arrange onion wedges. Drizzle 1 tablespoon of the oil over onion. Bake 5 minutes.
Meanwhile, cut eggplant half crosswise into 3/8-inch-thick slices. Cut top and bottom from red and yellow bell peppers; reserve for another use. Remove seeds and membranes from bell peppers. Cut into 2x1-inch strips.
Remove onion from oven. Arrange eggplant, bell peppers and mushrooms in pan; drizzle with remaining 3 tablespoons oil.
Return to oven; bake 10 minutes. Turn vegetables; bake 5 to 10 minutes longer or until vegetables are crisp-tender.
In medium bowl, mix all marinade ingredients. Add roasted vegetables; toss to coat. Cover; refrigerate until chilled, about 1 hour.
To serve, drain vegetables, discarding marinade; arrange on serving platter.